The aim of audio reproduction systems is to provide a high quality listening experience and clearly an important component of such a system is the sound delivery system which converts electrical signals to acoustic energy. Acoustic horns are well known sound delivery systems that function to increase sound output by appropriate loading of an electrically stimulated driver unit with the added ability to wholly or partly control the spatial distribution of sound generated by the driver. Typically, an acoustic horn utilises outwardly flaring walls to provide an expanding passage for the acoustic pressure wave between a throat entrance and a mouth exit. The acoustic horn is stimulated by the source driver unit located at the throat entrance which produces the acoustic pressure wave.
Whilst the wall profile of the acoustic horn may be axis-symmetric with an associated cross sectional area that is circular, in many cases the wall profile will have separate horizontal and vertical profiles and associated cross sectional areas which may be elliptical, or rectangular, or develop from one sectional area shape to another as one progresses from the throat to the mouth of the horn. This allows for the design of acoustic horns with well defined beamwidth that will suit a given environmental requirement. The beamwidth, also referred to as the angle of coverage, is defined as the angle formed between the −6 dB points of sound pressure level in the far field as calculated with respect to the central axis reading. As an example, a given environmental requirement might include an acoustic horn having an angle of coverage of 90° in the horizontal plane by 40° in the vertical plane, or 60° by 40°, and so on. Generically acoustic horns of this nature are called constant directivity horns and may be used individually or incorporated into arrays to provide an extended angular coverage.
As such, two important features of an acoustic horn loudspeaker are the beamwidth, whether it be in one or two sectional planes, and the spectral content of the output pressure wave produced by the acoustic horn. Clearly, for asymmetric sound fields independent beamwidths can be determined for different planes of symmetry, in the process independently defining the width of the angle of coverage or the height of the angle coverage of the sound field produced by the acoustic horn. The beamwidth parameter of an acoustic horn also quantifies the amount of sound energy that is transmitted to off-axis regions where the central axis of a horn will be determined by the horn geometry.
With regards to the spectral content of the output pressure wave produced by the horn, this in principle should closely mimic the associated spectral content of the electrical input signal to the driver unit. Another important feature of an acoustic horn related to the beamwidth and spectral content of the output pressure wave is the variation of the beamwidth of the acoustic horn with frequency. Ideally, there should be no variation with frequency; otherwise the spectral content of the sound will vary depending on the location of a listener with respect to the central axis of the acoustic horn.
As is well known, the most important free parameter that may be varied when designing an acoustic horn is the shape of the horn as this shape forms a surface that directs the acoustic pressure wave. Accordingly, one prior art approach in attempting to obtain a sound field of uniform intensity over a desired beamwidth is to join two horn sections with differing cross sectional area growth rates together. The first section, typically employing an exponential area growth rate, provides low frequency loading to the driver and its profile is used to control the width of the sound energy in one plane. At the intersection of the differing area growth rates (called the diffraction slot), sound is diffracted and the intersection essentially becomes a secondary “line source” of sound. The second section, usually employing a conical area growth rate then provides beamwidth control in the second plane. Further flanges can then be added to obtain control over “mid frequency beaming” (a narrowing of the beamwidth at intermediate frequencies) and furthermore vanes can be mounted in the throat of the horn to attempt to obtain control over “high frequency beaming”.
However, this approach has a number of serious disadvantages. Whilst a certain amount of control over the beamwidth can be achieved by use of a diffraction slot, this feature itself will also cause multiple reflections of sound waves within the horn, thereby resulting in an irregular frequency response which is easily measured and is perceived as colouration of the sound. Another significant disadvantage is that the sound emitted in the different planes will have different acoustic centres, these being defined by the respective centres of curvatures of the wavefronts of sound formed by the acoustic horn. Accordingly, acoustic horns based on the diffraction slot principle are difficult to incorporate into arrays where the alignment of individual horn components is an important consideration.
There have been a number of attempts to address the disadvantages of designs based on diffraction slots. One approach described in U.S. Pat. No. 6,059,069 relates to a loudspeaker horn having a straight wall section and a curved wall section. The straight wall section has diverging walls defining a coverage angle and the curved wall portion is connected to the straight wall portion at a point tangent thereto, and has a proximal end disposed perpendicular to the plane of the throat entrance. The diverging sidewalls define at least one coverage angle in orthogonal planes having a common apex in the plane of the throat entrance. Whilst this design is able to provide a common acoustic centre, thereby addressing one of the major disadvantages of diffraction slot designs, it only provides a relatively small amount of control of beamwidth in one axis as a function of frequency and this control is limited to only one axis.
US Patent Application No. 2003/0133584 employs an acoustic waveguide with a continuous least-energy-surface formed from an upper vertical control curve, a lower vertical control curve, right horizontal control curve and a left horizontal control curve. In addition, a circular throat end and a non-elliptical closed control curve form a mouth such that the continuous least-energy-surface is coincident with the six control curves. Again this design addresses the problem of providing a common acoustic centre but otherwise gives no guidance as to how a horn design having constant beamwidth as a function of frequency may be achieved.
In US Patent Application No. 2005/0008181, an acoustic waveguide is described that employs surfaces of constant coordinates in two coordinate systems. The coordinate systems chosen are those in which the equation that governs the propagation of sound either in the time domain (i.e. the wave equation,
                              ∇          2                ⁢                  p          ⁡                      (                                          x                →                            ,              t                        )                              -                        1                      c            2                          ⁢                                            ∂              2                        ⁢                          p              ⁡                              (                                                      x                    →                                    ,                  t                                )                                                          ∂                          t              2                                            =    0    ,where p({right arrow over (x)}, t) is the acoustic pressure at position {right arrow over (x)} and time t and c is the speed of sound) or alternatively in the equivalent frequency domain representation (i.e. the Helmholtz equation,
                              ∇          2                ⁢                  p          ⁡                      (                          x              →                        )                              -                                    ω            2                                c            2                          ⁢                  p          ⁡                      (                          x              →                        )                                =    0    ,where p({right arrow over (x)}) is the complex acoustic pressure at position {right arrow over (x)} and ω is the circular frequency) are separable and accordingly yield simplified solutions that depend on single coordinates. For example in a cylindrical coordinate system, a surface of constant radius forms a tube and the propagation of sound down the length of the tube can be considered to depend on axial position only, at least at low frequencies. Another simple example is a spherical coordinate system consisting of the coordinates (r, θ, φ) where r is the radius, θ is the azimuth angle and φ is the zenith angle. A surface of constant φ gives a conical horn, and at low enough frequencies the propagation of sound can be considered to depend on r only. The use of a prolate spheroid coordinate system allows independent control of beamwidth but requires a cylindrical vibrating surface as an input. An elliptical cylindrical coordinate system then provides a match between a flat vibrating surface and the prolate spheroid waveguide.
Whilst in principle, horns shaped according to solutions of the Helmholtz equation should result in the beamwidth being independent of frequency the necessarily finite termination of the horn at its mouth will result in diffraction and reflection of the acoustic wave as it leaves the horn. This results in a severe degradation of the performance of the acoustic horn with respect to the constancy of beamwidth with frequency. As is noted in US Patent Application No. 2005/0008181, the outward terminating edge of the horn may be flared to attempt to reduce the variation of beamwidth with frequency. However, this empirical approach does not provide a reliable or systematic method for designing horns that have a desired beamwidth variation as a function of frequency.
It is the object of the present invention to provide a method capable of designing a sound waveguide surface having improved directional characteristics.
It is a further object of the present invention to provide a method capable of designing a sound waveguide surface having a beamwidth that varies with frequency in a predetermined manner.